Plenty to do in Oak Bluffs and the logistics (parking, traffic etc) of taking the Island Queen are easier than Woods Hole. In addition, the Woods Hole ferry is run by a state agency and, as with all other infrastructure in the state, is suffering from neglect. They have canceled hundreds of trips since the beginning of the year due to maintenance issues. You may get there and not be able to get back.
The Island Queen is a private company not associated with the Steamship Authority.
The Flying Horses carousel, Mad Marthas ice cream, and the Tabernacle make OB a better destination than Vineyard Haven. The only attraction in Vineyard Haven is The Black Dog where you can buy an over-priced trendy tee shirt to prove that you've been there.